In 2011 NASA Langley completed the construction of a Hydro Impact Basin next to an existing gantry which allows testing of articles for water impact with both horizontal and vertical velocities. NASA Langley engineers use simulation in conjunction with the new testing facilities to evaluate water landings for space vehicles. To better understand Radioss’s Fluid Structure Interaction (FSI) capabilities, a blind benchmark of a water impact test of a 20 inch sphere was conducted. Both Arbitrary Lagrangian Eulerian (ALE) and Spherical Particle Hydrodynamics (SPH) were considered. Options for material modeling of the water were also evaluated. The Radioss results were then compared to existing test results looking at decelerations and pressure traces.

1. NASA Langley Radioss Benchmark
Water Impact of 20-inch Sphere
Antoine Segnegon - Altair
Innovation Intelligence® John Brink - Altair
Greg Vassilakos – NASA Langley
May 16, 2012

2. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.
Americas Solver Task Force
Formed in 2011 with the purpose of helping existing and potential
customers adopt and migrate to Altair Solver Technology
Team Members:
John Brink – Director – Radioss Bulk
Antoine Segnegon – Radioss Block Explicit
Andrew Dyer – MotionSolve
Jaideep Bangal – AcuSolve
Abigail Arrington – AcuSolve

3. Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.
Introduction
• In 2011 NASA Langley completed construction of a Hydro Impact Basin
• The Hydro Impact Basin is next to an existing gantry

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Introduction
• Allows water impact testing for both horizontal and vertical velocities
• Simulation is used in conjunction with the basin to evaluate water
landings for space vehicles

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Objective of Study
• NASA Langley engineers wanted to determine the suitability of Radioss
for water impact simulation
• Blind Prediction—test results not available until after simulation
completed
• The problem given was a 20-inch hemisphere dropped from 5 and 10 ft

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Responses of Interest
• Deceleration of the sphere
• Pressure on the surface of the sphere
Pressure Transducer Locations
Test Set-Up
Test Article Configuration

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Options to Consider for Radioss Set-up
For Fluid Structure Interaction (FSI) simulation in Radioss, the following
options and parameters need to be considered:
• ALE or SPH
• Mesh Density and Spatial Discretization
• Material Modeling
• Initial Conditions
• Boundaries
• Gap of Interface Between Structure and Fluid
• Stiffness of Interface Between Structure and Fluid

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Initial Set-up for Blind Prediction
• ALE (Arbitrary Lagrangian Eulerian) formulation used
• Expected to be less CPU costly based on experience
• Mesh Density and Spatial Discretization
• Followed guidelines from experience for mesh density and relative mesh sizing
• Sphere is modeled 20 mm x 20 mm
• Fluid mesh is modeled to have surface half the size of the sphere and half again
through the thickness
Lagrangian
5 ALE

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Initial Set-up for Blind Prediction
• Material Modeling
• Choose Law 51 (Multi-Material Solid, Liquid, Gas) for fluid which exhibits less
diffusivity than older multi-material models
Rigid
Air Infinite
Sphere
Boundary
• Use Law 1 (Linear Elastic) for sphere
• Initial Conditions Air Elements
• Added gravity to the problem with /GRAV
• Did not include initial hydrostatic pressure
• Boundaries
Water Elements
• Used quarter model with symmetries
• Used infinite boundary on top of air surface
Quarter Model
~900k elements

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Initial Set-up for Blind Prediction
• Gap of Interface Between Structure and Fluid
• Use Type 18 interface (Penalty-stiffness based, coupled Eulerian-Lagrangian) with
recommended Gap = 1.5 Lc, where Lc is the characteristic length of the fluid
(Lc=Volume/(Largest surface Area)=10x10x5/10x10)
• Stiffness of Interface Between Structure and Fluid
• The interface stiffness, Stfac, changes with mesh size and velocity and is
recommended to be:
Stfac = (ρ * V2 * Sel)/GAP
ρ - the highest fluid density in the model
V – impact velocity of the Lagrangian mesh
Sel - Surface area of Lagrangian impact element

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Animation of Results – 5 Foot Drop
Animation of Results

12. Observation Regarding Fluid Mesh for ALE
The finite element model representing the entire cylindrical tank showed
unacceptable behavior at the contact area (top row of pictures) with the first
mesh attempted. The mesh of the fluid was redone in the impact area to avoid
“butterfly” mesh. The second row of images show the re-mesh fixed the issue.
Leakage at
“Butterfly” Interface
mesh
Cleaner
Response
Orthogonal
mesh

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Comparison to Test Results – 5 Foot Drop
• Accelerations match the test results very closely
• The pressure trace does not capture the test magnitude
• The simulation ran for just over 2 hours on a 16-CPU machine
Note: Test acceleration results shown have been filtered with a 180 Hz Butterfly Filter; there is no filtering of simulation results
The pressure gage data from the tests is unfiltered except for the high frequency (4300 Hz) analog anti-aliasing filter that exists in the Data Acquisition System

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Comparison to Test Results – 10 Foot Drop
• Again, accelerations match the test results very closely
• The pressure trace does not capture the test magnitude
• The reduced model ran in 45 minutes on a 24-CPU machine (4x faster)
Note: Test acceleration results shown have been filtered with a 180 Hz Butterfly Filter; there is no filtering of simulation results
The pressure gage data from the tests is unfiltered except for the high frequency (4300 Hz) analog anti-aliasing filter that exists in the Data Acquisition System

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Investigation of Pressure Profile
• J.P.D. Wilkinson’s paper “Study of Apollo Water Impact, Final Report,
Volume 4, Comparison with Experiments”, May 1967
• Pressure profile for water impact shows very steep pressure gradient
and very localized effect

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Refine Mesh of 5 Foot Drop
• The mesh is refined so that the sphere has ~10mm x 10mm elements
• The fluid mesh is also refined to maintain proper mesh size ratios to
5mm x 5mm and 2.5mm in the impact direction

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Mesh Refinement Results
• Accelerations peaks match well
• Pressures improve but at larger CPU cost (X 5)
Note: Test acceleration results shown have been filtered with a 180 Hz Butterfly Filter; there is no filtering of simulation results
The pressure gage data from the tests is unfiltered except for the high frequency (4300 Hz) analog anti-aliasing filter that exists in the Data Acquisition System

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Conclusions
• Doing a blind prediction resulted in very good correlation using
standard established modeling practices for Radioss FSI
• Pressure traces, however show a lower peak magnitude in simulations;
refining the mesh appears to help, but further refinement is necessary to
see if the pressures converge to test results
• It is unclear whether it is necessary to capture the exact pressure trace
to model the structural response of the test article
• Including the effect of the tank in the simulation was unnecessary; using
infinite boundaries gave very similar results at much less cost
• It is recommended to use an orthogonal mesh in the fluid in the area of
impact to avoid fluid to structure contact issues
• Radioss can be used confidently for water impact problems